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The Scanning Electron Microscope (SEM) image of an ant, as demonstrated for a high-school outreach program, has inspired Assistant Professor Jun Yao of the Electrical and Computer Engineering Department at the University of Massachusetts Amherst to develop bioinspired, ultrasensitive pressure and strain sensors using microparticles resembling the bristles or tactile hairs ubiquitous in insects. “My hope was that the extremely enlarged image of an ant, perhaps a bit scary and monster-like, would excite the students’ interest in the ordinarily invisible nanoscale domain,” says Yao. Much more than that, the demo inspired Yao’s own subsequent research and his resultant paper in the prestigious journal Nature Communications on the 4th of December 2018.

The excellent sensing performance of these ant-inspired pressure and strain sensors demonstrates their great potential in wearable technologies (e.g., for health monitoring) and other human interfaces with improved signal and comfort.

As Yao and his fellow authors (Bing Yin, Xiaomeng Liu, Hongyan Gao, and Tianda Fu) describe their new technology in their Nature Communications paper, “We mimic these [insect] features by using synthetic zinc-oxide (ZnO) microparticles, each having spherically-distributed, high-aspect-ratio, and high-density nanostructured spines resembling biological bristles. Sensors based on thin films assembled from these micro-particles…show supreme overall performance.”

Yao remembers how this innovation got its start. “As a newly established research group, we faced the typical challenge of how to get our work off the ground with limited resources. With a lab renovation and equipment installation still in progress, it took some head-scratching to come up with experimental options in the early days.”

At this time Yao’s postdoc Bing Yin, armed with only the most basic tools at hand – a beaker and a hotplate – found that by mixing sodium hydroxide and zinc acetate dihydrate solutions at a temperature of 40 °C he could produce many synthetic ZnO microparticles, each featuring a forest of high-density nanostructured spines.

“The microparticle resembles a miniaturized sea urchin (about 10,000 times smaller than life-size),” recalls Yao. “I thought the structures were interesting but did not have a concrete idea as to how they could be utilized.”

At the time, by coincidence, Yao and his students were hosting a summer visiting program. “We invited high-school students from a local public school to our lab,” says Yao, “in an effort to promote science, technology, engineering, and mathematics among pre-college students.”

As part of this program, Yao’s team captured some ants and showed them under the SEM. “In fact, not only did the students show great interest, but I myself was unexpectedly intrigued by the microscopic features of the ant,” says Yao. “Although I knew in a general sense that insects can have tactile hairs as mechano-sensory organelles, it was the first time I had visualized the fine structures in these hairs under the SEM.”

This experience marked a moment of truth for Yao. “Instantly, their tapering geometry reminded me of the uncannily similar nanostructured spines covering the ZnO microparticles that Bing had synthesized,” says Yao. “We naturally leapt quickly to the possibility of functional emulation, especially the potential for improvement in sensor design, as we knew that often nature had already optimized the functionality.”

A quick literature search taught Yao and his lab students that the tapering spines (or bristles) in insects not only serve as a lever arms to promote mechanical-signal transduction, but also a clever strategy to protect it from mechanical breakage. Yao says the hierarchical distribution over the body further improves the multi-directionality of signal detection.

“We hypothesized that a thin-film sensor made from ZnO microparticles, covered with spines having both geometric and distributional similarities to the bristles in insects, could yield high sensitivity as well as durability,” Yao says.

Yao explains that the subsequent fabrication of the thin-film sensors was relatively straightforward, but the result surprised his lab. The biomimicry led to vastly superior overall performance in the sensors, with a ten-fold improvement in key parameters (e.g., sensitivity, gauge factor, pressure detection limit) in comparison with many conventional mechanical sensors.

As the researchers conclude, “Other properties, including a robust cyclability larger than 2,000, fast response time of about 7 ms, and low-temperature synthesis compatible to various integrations, further indicate the potential of this sensor technology in applying to wearable technologies and human interfaces.” (December 2018)